The invention is based on a method for cutting flat work pieces made of a brittle material, in particular glass or ceramic, in which a laser beam having a linear beam profile followed by a cold spot is moved along a dividing line having a specified contour. A preferred application of the method is in the cutting of flat glasses.
The invention is based further on a device for cutting a work piece of this type using a laser beam in the form of a linear beam profile followed by a cold spot.
Conventional methods for cutting flat glasses are based on the use of a diamond or a small rotary cutter to first produce a scribed line in the glass in order to then break the glass by application of an external mechanical force along the weak point produced in this fashion (xe2x80x9claser scribe and breakxe2x80x9d). The disadvantage of this method is that the scribed line causes particles (fragments) to be released from the surface, which said particles can deposit on the glass and scratch it, for example. Additionally, xe2x80x9cchipsxe2x80x9d can be created in the cut edge, which results in an uneven glass edge. Furthermore, the micro-cracks produced in the cut edge during the scribing process lead to reduced mechanical stressability, i.e., to increased risk of breakage.
An approach for preventing the formation of fragments as well as chips and micro-cracks is to cut glass based on thermally induced mechanical tension. In this approach, a heat source directed at the glass is moved at a fixed speed relative to the glass, thereby producing such a high thermal mechanical tension that cracks form in the glass. Infrared emitters, special gas burners and lasers, in particular, possess the necessary property of the heat source to position the thermal energy locally, i.e., with an accuracy of greater than one millimeter, which corresponds to typical cutting accuracies. Lasers have proven to be effective and have gained acceptance due to their good focusability, good controllability of output, and the ability to shape the beam and, therefore, to distribute intensity on glass. As a result, the glass can first be scribed using the laser beam and then broken mechanically. Or, the glass can be separated directly using the beam in conjunction with a mechanically-applied starting fissure, i.e., it can be cut. The terms xe2x80x9cseparate/dividexe2x80x9d or xe2x80x9ccutxe2x80x9d are intended to encompass the term xe2x80x9cscribe-breakxe2x80x9d as well as xe2x80x9ccutxe2x80x9d.
This laser beam separating methodxe2x80x94which induces a thermal mechanical tension up to above the breaking strength of the material by means of local heating using the focused laser beam in conjunction with cooling applied from the outsidexe2x80x94has been made known in numerous publications, e.g., in EP 0 872 303 A 2.
The laser beam separating methods mentioned hereinabove differ in particular by the configuration of the focal point. The method according to DE 693 04 194 T 2, for example, uses a laser beam having an elliptical cross section followed by a cold spot.
The publication EP 0 872 303 A 2 cited hereinabove describes a laser beam separating method that provides a focal point having a U-shaped or V-shaped contour that opens in the direction of separation. Contours derived from these, such as X-shaped focal points, are also described. In both cases, the laser beam focal points have a two-dimensional structure that has proven effective in accomplishing straight cuts. When making freeform cuts, a curved, two-dimensional focal point adapted to the contour of the dividing line would have to be produced and moved along the contour, including the cooling that follows it. This would require, in particular, coupling the scanner device producing the respective two-dimensional focal point plus the cold spot device to a trajectory control device, the realization of which is very problematic due to the large quantities of data to be exchanged and the cutting speeds required.
A laser beam separating method has been made known in DE 43 05 107 C 2 in which the laser beam is shaped in such a fashion that its beam cross-section has a linear shape on the surface of the work piece, and in which the ratio of length and width of the impinging beam cross-section can be adjusted using an aperture in the laser beam path. This method is greatly restricted as well in terms of its usability. It cannot be used to make freeform cuts and, because the cooling is not to be applied until after the dividing line has been heated completely, e.g., using a jet of cold compressed air, the known method is suited practically only for use as described to cut off the extruded rim of hollow glassware, in which method the hollow glassware rotates in the stationary laser beam, whereby the rim is first heated all the way around its circumference by means of the laser beam and then cooled in supportive fashion by blowing off the gas.
The invention is based on the object of carrying out the method described initially in such a fashion, and of designing the associated device in such a fashion that freeform cuts can be accomplished relatively easily using the laser beam separating method.
Based on the method for cutting flat work pieces made of a brittle material, in which a laser beam having a linear beam profile followed by cold spot is moved along a dividing line having a specified contour, the object is attained according to the method by the fact that, to make freeform cuts, a linear focal point having significant intensity increases at both ends is produced on the work piece, in which said method at least 60% of the beam output is concentrated at the ends of the linear focal point, and which said focal point is moved along the dividing line in such a fashion that, even when the dividing line is curved, the two ends lie on the dividing line, and the length of said linear focal point is adjusted as a function of the curvature of the contour of the dividing line.
As a result of these measures, it is possible to accomplish freeform cuts having any shape using a relatively simple method, because the greater part of the energy is concentrated in the ends of the focal point, and this said energy is confined to the dividing line while the length of the focal point is adapted to the curvature of the contour, so that, when small radii of curvature are involved, the length of the focal point is correspondingly short and, when large radii are involved, the length of the focal point is correspondingly long, in order to ensure the necessary application of energy to the dividing line.
To prevent the work piece material from melting, the method is carried out according to one embodiment of the invention in such a fashion that the laser output is adjusted as a function of the length of the linear focal point.
The method can be carried out relatively easily and advantageously according to another embodiment when the linear focal point is produced by scanning the laser beam.
With regard for the device for carrying out the method according to the invention, the object is attained by the fact that an optical system for producing a linear focal point having significant intensity increases at both endsxe2x80x94in which said ends at least 60% of the beam output is concentratedxe2x80x94is provided, which said optical system is coupled to a numerical trajectory control device via a profile control device in such a fashion that, even when the dividing lines are curved, both ends of the linear focal point are positioned on the dividing line of the freeform cut, and the length of the focal point is adjusted as a function of the curvature of the contour of the dividing line.
Due to the linear focal point, the optical system can be designed relatively simple in nature, and conventional trajectory control devices can be used to move the ends of the focal point on the dividing line and adjust the length of the focal point to the contour of the dividing line.
An optimal separation is ensured when, according to one embodiment of the invention, further axes coupled to the trajectory control device are provided to position the cold spot in relation to the linear focal point and the work piece.
According to a further embodiment of the invention, the device can be designed particularly simple in nature when the optical system comprises a scanner having an oscillating mirror. To change the length of the linear focal point, one only need to change the oscillation amplitude of the oscillating mirror so that the profile control device and the trajectory control device can be designed relatively simple in nature.
Further features and advantages of the invention result from the description of an exemplary embodiment shown in the drawing.